Subj : Re: Reversing a number
To   : comp.programming
From : Gerry Quinn
Date : Tue Sep 13 2005 05:12 pm

In article <4326BFE2.2775@mindspring.com>, pfiland@mindspring.com 
says...
> Rob Thorpe wrote:
> > 
> > Willem wrote:
> > > Roger wrote:
> > > ) "Irrwahn Grausewitz" <irrwahn35@freenet.de> wrote in message
> > > ) news:shv7i1p3on0qpsr2rumeifst69o8tjtp32@4ax.com...
> > > )> But why treat the first iteration as a special case?
> > > )> Just replace the two lines above with:
> > > )>
> > > )>     int n = 0;
> > > )>
> > > )>>    while (i)
> > > )>>    {
> > > )>>        n *= 10;
> > > )>>        n += i % 10;
> > > )>>        i /= 10;
> > > )>>    }
> > > )>>    return n;
> > > )>>}
> > > )
> > > ) Then reverse(10000) is (1).
> > >
> > > Of course it is; the function returns an integer, after all.
> > 
> > I think the OP means a number in the mathematical sense of the word.
> > ie:-
> > 
> > 1 -> 1
> > 21 -> 12
> > 5634 -> 4365
> > 
> > As Jerry Quinn pointed out 
> > it's simplest to convert it to a string then
> > reverse it and convert it back again.
> 
> #include <stdlib.h>
> #include <limits.h>
> 
> int revdecimal(int number);
> void itoa_r(int n, char *s);

Depends on the language.  E.g. Blitz Basic 3D:

Function GetReverse( x )
	a$ = x
	For i = Len( a$ ) To 1 Step -1
		b$ = b$ + Mid$( a$, i, 1 )
	Next
	return b$
End Function 

(This language can automatically cast between integers and their 
decimal string representations.)


- Gerry Quinn


> int revdecimal(int number)
> {
>     char string[sizeof number * CHAR_BIT / 3 + 1];
>     
>     itoa_r(number, string);
>     return atoi(string);
> }
> 
> void itoa_r(int n, char *s)
> {
>      int tenth, min_offset;
>      char *p;
>      
>      min_offset = 0;
>      if (0 > n) {
>          if (-INT_MAX > n) {
>              ++n;
>              ++min_offset;
>          }
>          n = -n;
>          *s++ = '-';
>      }
>      p = s;
>      tenth = n;
>      do {
>          tenth /= 10;
>          *p++ = (char)(n - 10 * tenth + '0');
>          n = tenth;
>      } while (tenth != 0);
>      *s = (char)(*s + min_offset);     
>      *p = '\0';
> }
> 
>

.